The invention relates to an anesthesia monitor, and more particularly to an anesthesia monitor for analyzing continuously a part of the inspiration and expiration of a living body so as to perform movement analyses about the respiration, the circulation and the metabolism thereof.
Such anesthesia monitors analyze concentrations of N.sub.2, O.sub.2, CO, CO.sub.2, N.sub.2 O and added anesthetics involved in the inspiration or expiration of a patient in order to keep well, i.e., satisfactory, the patient's condition. Normally, an inhalation anesthesia employs nitrous oxide (N.sub.2 O). Further, anesthetics such as halothane in which a hydrogen atom of alkane or ether is substituted by a halogen atom are recently employed. Thus, the variety of anesthetics used is increasing. It is required to perform the concentration analyses of such anesthetics comprising many components. The conventional anesthesia monitors have normally utilized an infrared spectroscopy or a mass spectrometry as a analysis method of the anesthetic concentration involved in the expiration or the inspiration. The mass spectrometry usually employs a uni-convergence method or a quadruple-pole method to analyze the expiration or the inspiration.
As described above, since the variety of anesthetics tends to be increased, the anesthesia monitor is required to possess the following performance characteristics. First, one requirement is that the anesthesia monitor is able to make a measurement of various components involved in the expiration and the inspiration of the patient. Second, one requirement is that the anesthesia monitor is able to make a separation and a determination of components which have analogous properties to one another. Third, one requirement is that the anesthesia monitor is able to make a measurement of many components in real time. Fourth, one requirement is that the anesthesia monitor has a suitable size for an operation room, and in addition and ease of installation and operation thereof.
It will be investigated whether or not the above and prior known analyzing methods of the concentration comply with the above requirements.
The infrared spectroscopy has the following problems in a separation of the concentration of mixed gases. A first problem is that there exist many components of gases whose absorption spectrums overlaps with one another. For example, with respect to N.sub.2 O and CO.sub.2 being important as analyzing objects, N.sub.2 O has an intensive spectrum peak in the range of the wavelength from 4.4 micrometers to 4.6 micrometers. In contrast, CO.sub.2 has an intensive spectrum peak in the range of the wavelength from 4.2 micrometers to 4.4 micrometers. Thus, the intensive spectrum peaks of N.sub.2 O and CO.sub.2 overlap with one another in the vicinity of a wavelength of 4.4 micrometers. A normal anesthesia performed on condition of a high concentration of N.sub.2 O and a low concentration of CO.sub.2 forces the measurement results to have an error. A second problem is that the molecular collision causes a spectral line to be broad. In mixed gases, molecules of gas as a measuring object makes collisions with other molecules thereby causing an interchange of molecular energies. The molecular energy is varied according to the molecular weight and the dipole moment of the collision gas. As a result, the infrared absorption band becomes broad and the dummy absorption wavelength is also influenced. Thus, an error in the measurement results appears depending upon the circumstances of other components in the mixed gases. The third problem is that it is frequently difficult to make a measurement of halide anesthetics which are recently developed. The halide anesthetics have intensive absorption lines in the vicinity of a wavelength of 3.1 micrometers respectively. This makes it impossible to distinguish individuals of the halide anesthetics. A fourth problem is that the infrared spectroscopy is unable to make a measurement of components which are subjected to no variation of its dipole moments by a molecular oscillation such as a diatomic molecule. Thus, it is impossible to measure N.sub.2 O and CO.sub.2 and the like as serving basic components on an artificial respiration.
The mass spectrometry has the following problems. It is difficult to make a separation measurement of the component of any gases as measuring objects. Foe example, nitrous oxide (N.sub.2 O) which is frequently used for a general anesthesia has a molecular weight of 44.001. Carbon dioxide (CO.sub.2) which frequently becomes an analyzing object for the respiration analysis has a molecular weight of 43.990. Both molecules have the same integer mass number. The difference in the precise mass number thereof is only 0.11 amu. The accomplishment of a direct separation determination of the above molecule components by measuring their molecular peaks thus requires a resolution of much more 10000. The resolution is defined by EQU Resolution&gt;&gt;mass number/(peak interval/2)=8000.
The conventional mass spectrometry utilizing the mono-convergence system or the quadruple-pole system has a resolution of only approximately 1000. Thus, it is difficult to make directly the separation measurement. As a result, the conventional anesthesia monitor unwillingly uses the following methods.
In a first method, fragment peaks of all components expected to be involved in the mixed gases are compared, after which peaks, or uni-peaks which do not overlap with one another are selected. The height of the selected peak is regarded as a quantity of a component of the above mixed gases.
There exists a second method utilizing a multiple regression method which solves simultaneous equations or normal equations by the least square method. In the simultaneous equations, patterns coefficients of all components expected to be involved in the mixed gases are so utilized that unknown variables are the concentrations of the above components.
With respect to the first method, the separation measurements of N.sub.2 O and CO.sub.2 are unable to use molecular peaks having maximum intensity respectively. The separation measurement of N.sub.2 O uses a fragment peak of M/z 30. The separation measurement of CO.sub.2 uses a fragment peak of M/z 12. As is well known, such fragment peaks have an intensity that are considerably lower than that of the molecular peaks as base peaks. This causes the accuracy of the measurement to be lowered.
With respect to the second method, as it is known, when there exists an unexpected component in a material gas or a ratio of measurement peak signal to a noise is not sufficiently large, an unexpected large error occurs.
Further, there exists a gas chromatograph mass spectrometry method. In this method, the gas chromatograph used for the normal organic analyses are as previously prepared. The components of the mixed gases are separated according to specific holding times, followed by the mass analysis. Since such method has an analyzing period in the range from several minutes to several ten minutes, the above mentioned real time measuring condition is not satisfied and thus the method is not applicable to the patient expiration gas measurement application.
In addition, there exists a Fourier transformation mass spectrometer that is commercially available. The applicability of the method to the patient gas expiration and inspiration analyses is disclosed in Anal. Chem. 60, pp. 341-344, 1988. Disclosed therein is a large and extremely expensive apparatus utilizing a super-conductance magnet, which have not been applied to a general clinical application.
Furthermore, there exists a large bi-convergence mass spectrometer possessing a resolution more than 10.sup.4. Although such spectrometer is capable of making the separation measurement of N.sub.2 O and CO.sub.2 by using those molecular peaks, that is practically unsuitable for the anesthesia monitoring in that it has a large installation area (footprint), a complicated manner of operations, a long measuring time at a high resolution. The high price thereof also is bar to its application in a patient anesthesia monitoring application.